N. Mrosovsky

Phase response curves for social entrainment

SummaryPhase shifts in free-running activity rhythms of male golden hamsters,Mesocricetus auratus, often occur when they establish a new territory and home after a cage change. Similar shifts also often occur after pairs of animals interact with each other for half an hour. When these events take place during the middle of the hamsters subjective day, they produce phase advances: when late in the subjective night, they produce phase delays. Repeated social interactions at the same time of day can entrain activity rhythms in a way consistent with the shape of the phase response curves. Not all individuals become entrained, as is predictable from the modest amplitude of the phase response curve. The effects of social interactions and of other disturbances may be mediated through an oscillator phased by general arousal. The present findings have implications for the interpretation of drug-induced changes in biological rhythms.

Behavioural entrainment of circadian rhythms

This paper reviews the discovery and characterization of a behavioural system for entrainment of circadian rhythms. This behavioural system depends on non-photic inputs but interacts with the light-entrainment system. Non-photic stimuli can be powerful quantitatively: behavioural events can shift rhythms by several hours. Nonphotic entrainment offers scope for rephasing biological rhythms in circumstances where light input from the environment is inadequate.

Running activity mediates the phase-advancing effects of dark pulses on hamster circadian rhythms

SummaryPulses of darkness can phase-shift the circadian activity rhythms of hamsters,Mesocricetus auratus, kept in constant light. Dark pulses under these conditions alter photic input to the circadian system, but they also commonly trigger wheel-running activity. This paper investigates the contribution of running activity to the phase-shifting effects of dark pulses. A first experiment showed that running activity by itself can phaseshift rhythms in constant light. Hamsters were induced to run by being confined to a novel wheel for 3–5 h. When this was done at circadian times (CT) 0, 6, and 9, the mean steady-state phase-shifts were 0.6 h, 3.5 h, and 2.3 h, respectively. The latter two values are at least as large as those previously obtained with dark pulses of similar durations and circadian phases. A second experiment showed that restricting the activity of hamsters during 3-h dark pulses at CT 9 reduces the amplitude of the phase-shifts. Unrestrained animals phase-advanced by 1.1 h, but this shift was halved in animals whose wheel was locked, and completely abolished in animals confined to nest boxes during the dark pulse. Activity restriction in itself (without dark pulses) had only minimal phase-delaying effects on free-running rhythms when given between ca. CT 10 and CT 13. These results support the idea that, in hamsters at least, dark pulses affect the circadian system mostly by altering behavioural states rather than by altering photic input to the internal clock.

Differential effects of dark pulses on the two components of split circadian activity rhythms in golden hamsters

SummarySingle 6-h pulses of dark were presented to golden hamsters displaying split circadian activity rhythms in constant light. The differential responses of the split components immediately following a 6-h dark pulse suggests that each component has a separate and similar bidirectional phase response curve, similar in shape to that described by other investigators for non-split hamster activity rhythms. The steady-state phase shifts of the split components were more variable with accompanying changes in period, changes in the time-interval between components, and the presence of transients, reflecting the gradual re-establishment of the pre-pulse 180 ° phase relationship. In addition, dark pulse presentation sometimes resulted in the replacement of a single split component by a number of smaller, frequent bursts of activity; this indicates that there may be a population of tightly coupled oscillators underlying each split component.

Infradian cycles in dormice (Glis glis)

SummaryInfradian cycles of body weight occur in captive dormice,Glis glis. The period of these cycles is often about 2 months but is highly variable both between and within individuals. These cycles can persist for at least 3 years. This paper describes a number of variables that change on an infradian basis along with body weight. These include: food and water intake, weight of the liver, adrenal and salivary gland weight, testes weight and spermatogenic condition, body temperature and wheel-running activity. Both the amount of running and its distribution over a 24 h period change. However, infradian cycles do not appear to be generated by the circadian system because they persist even in continuous bright light sufficiently intense to disrupt circadian organization. They also persist in LD 18∶6, LD 12∶12, LD 6∶18 and in naturally changing photoperiods. No evidence of photoperiodic effects on cycles was detected except that animals in long days have slightly higher moulting scores than those in short days. Moulting is more marked when body weight stays relatively stable. In dormice remaining stable in weight for several months testes have sperm present. The period of infradian cycles is temperature dependent: in a cool room animals that hibernate have longer cycles with periods varying from a few months to about a year. In a number of respects at least, infradian cycles appear to be accelerated circannual cycles. The adaptive value of a system that gives rise to infradian cycles probably relates to opportunism in an unpredictable environment. Infradian cycles in dormice may be a useful preparation for obtaining data on spontaneous physiological and behavioural changes in hibernators in less than a year.

Circannual cycles in golden-mantled ground squirrels: phase shift produced by low temperatures

SummaryGolden-mantled ground squirrels,Spermophilus lateralis, were kept in a warm room (21 °C) until they had begun to lose weight following attaining a peak of body weight in the autumn. An experimental group was then transferred to a cold room (−3 °C) and kept there for about 9 months before being returned to the warm room. A control group was kept in the warm room throughout. The peaks in body weight of animals that had been exposed to the cold occurred 4.5 months later than those of the control animals. This difference was maintained when the animals were studied for a further year. Reproductive condition in the experimental animals, after they had been returned to the warm room, also occurred several months later than that in control animals. An extended sojourn in the cold therefore results in a phase shift in circannual cycles of this species.

Circannual cycles in golden-mantled ground squirrels: Experiments with food deprivation and effects of temperature on periodicity

SummaryFemale golden-mantled ground squirrels,Spermophilus lateralis, kept in a cool room (9.5 °C), were food deprived and their hibernation thereby extended by 11 weeks on average compared to undeprived control animals in the same room. The deprived group was then refed. Despite their prolonged hibernation, peak weights and onset of hibernation in the subsequent season were only 2–4 weeks later than in the undeprived controls and this effect was only marginally significant. However, the periods of circannual cycles of both the deprived and undeprived groups in the cool room were significantly longer than those of another group of female ground squirrels kept in a warm room (21 °C). It is argued that warm temperatures are capable of terminating the winter phase of circannual cycles, namely that of hibernation and declining body weight. If animals are kept continuously in a cool environment this phase ends spontaneously. Even many weeks of food deprivation are only capable of extending this phase by a relatively small amount, if at all.

Synchronization of circannual cycles: a cold spring delays the cycles of thirteen-lined ground squirrels

SummaryGround squirrels, show circannual cycles with periods normally less than a year when kept under laboratory conditions. The way in which environmental factors synchronize these cycles with the geophysical year under natural conditions is not known. We tested the possibility that cold temperatures can cause long-term phase delays in circannual cycles of thirteen-lined ground squirrels.Two groups of animals were kept in the cold (4 °C) for either 8.5 or 13 months, after which they were returned to the warm (21 °C) and kept there until they had completed at least one additional cycle. A third group was kept in the warm for the entire experiment. Most of the males in the cold room groups became arrested in the spring phase of their cycles while they were in cold. When returned to the warm, these males resumed cycling. When animals showed prolonged spring phases, their cycles were phase-delayed and continued to reflect this delay even after they were returned to the warm. Both body weight cycles and molt cycles were delayed. In contrast to the males in the cold room, none of the females in the cold room groups and none of the warm room animals of either sex showed this response.Our results demonstrate that cold temperatures can phase-delay both body weight and molt cycles and that the spring phase is a critical stage in this effect. We suggest that spring temperatures are largely responsible for the seasonal synchronization of circannual cycles in ground squirrels and are, therefore, a possible Zeitgeber for these cycles.

Circannual cycles in golden-mantled ground squirrels: Lengthening of period by low temperatures in the spring phase

SummaryTwo complementary experiments were designed to study the effect of cold temperatures on the spring phase of the circannual cycle of golden-mantled ground squirrels (Spermophilus lateralis). In the first experiment, a group of animals was kept in a warm room (22 °C) until the latter half of their weight-loss hibernation phase. They were then placed in a cold room (3 °C) for about 2 1/2 months, after which they were returned to the warm room. The control group was kept in the warm room throughout the experiment. The exposure to cold delayed both subsequent body weight peaks and the end of molt by about 6 weeks. In the second experiment, ground squirrels that had been transferred to a warm room (21 °C) from a cold room (6 °C) while still in their weight-loss hibernation phase reached peak body weights 6 weeks earlier than a group transferred after hibernation had ended. These results show that cold temperatures in the spring phase alone can produce a lengthening of the circannual cycle that is of similar magnitude to that found previously when animals are kept continuously in the cold.

Reproductive cycles in the male dormouse (Glis glis)

Abstract 1. 1. Dormice show short, endogenous cycles of body weight which have been suggested to be accelerated annual cycles; this implies that reproductive condition should vary with the body-weight cycle, because cycles of reproductive condition and body weight are typically phase-locked to each other in seasonal hibernators. 2. 2. Testes were examined at various phases of the short body-weight cycle. They appear to show cycles that are phase-locked to the body-weight cycle; variations in testes were related to the phase of the cycle rather than to the size of the animals. Larger testes and more sperm were found in the high-weight phase. 3. 3. These results are consistent with the idea that the short cycle is an abbreviated annual cycle, but the available data are not sufficient to relate this unusually short, endogenous cycle of reproductive condition to patterns of reproduction in natural conditions.